Apparatus and method for generating a screened reproduction of an image

ABSTRACT

An apparatus is disclosed for generating a screened reproduction of an image having a scanner and a CPU receiving input density values of an original and coordinate information and storing screening information for providing an operating input to a plotter enabling the plotter to provide a screened image of the original. The CPU is operative to modify at least one of the input density values, the coordinate information and the screening information, thereby to provide an operating input generally free of repeated artifacts arising from the lack of coordination between the coordinate systems of the plotter and those of the screening information. This is achieved, in particular, by maintaining a uniform number of plotter lines in each dot of the screened image for a given input density.

FIELD OF THE INVENTION

The present invention relates to screened image reproduction and moreparticularly to a method and apparatus for electronically generating ascreened reproduction of an image.

BACKGROUND OF THE INVENTION

Electronic screening for image reproduction is well known in the art.According to a well known technique described in U.S. Pat. No. 4,456,924of the present assignee, for each screened dot, a multiplicity ofcoordinates of a laser plotter are translated into screen-cellcoordinates. A corresponding cell memory is preloaded by thresholdvalues, defining a cell memory matrix. Input digitized scanned densityvalues of an image, such as a color separation, are compared with thethreshold values, cell by cell. The results provide an on/off controlinput for a laser plotter.

U.S. Pat. No. 4,635,131 describes a method of and apparatus forproducing halftone dot film of graduated density distribution. In anelectronic image reproduction system, a halftone dot film of a densitydistribution of a specific pattern is produced by obtaining a value 1expressed by an equation 1=f(x)+g(y) representative of the specificpattern corresponding to the density value. By superimposing a densitysignal corresponding to the value 1 on an image signal developed byscanning an original, a reproduction image of the original, modulated bythe density signal of a specific pattern, is obtained.

U.S. Pat. No. 4,825,298 to Ikuta and Murai describes a technique forgenerating a screened reproduction of an image in which the densitydistribution of a given screen dot is expressed in three dimensions,wherein the area of the screen dot is expressed along X and Y axes andthe density is expressed along a Z axis perpendicular thereto. A filmcoordinate generator generates film coordinates (u,v), corresponding tothe position of an exposure beam on a recording film which position isdetected by encoders. The film coordinates are in turn supplied to ascreen coordinate generator to be converted into virtual screencoordinates (x,y). A beam control signal generator receives thecoordinates (x,y) and an image signal corresponding to the position ofthe exposure beam to output a beam control signal indicating lighting ofthe exposure beam when one of the coordinates (x,y) is between upper andlower limit values, corresponding to the same, which are previouslydetermined for each combination of the other of the coordinates (x,y)and the density value of the image signal.

U.K. Published Patent Application 2,157,119A to Ikuta describesapparatus which operates similarly to the technique of U.S. Pat. No.4,456,924 but does not employ a matrix memory. Instead, the thresholdfunction is calculated on the fly in real time or near real time. Thisapparatus is limited to relatively simple dot configurations.

U.S. Pat. No. 4,556,918 describes a method and apparatus for generatingscreened halftone images which includes means for assuming an area ofhalftone dots with desired periodicity and tone reproducibility,subdividing the area into minute cells and setting address values foreach of the minute cells, computing out a threshold value of density foreach of the cells as a function of the relevant address values and usingthe computed value as a threshold value of density for the cell,apparatus for obtaining a density-related video signal of the portion ofthe original corresponding to each of the cells by scanning theoriginal, and apparatus for producing halftone dot signals by comparingthe video signals and the threshold value of density with each other.

Applicant/assignee's earlier U.S. Pat. No. 5,079,721 describes apparatusand a technique for generating a screened reproduction of an imageincluding the steps of providing a representation of an original havinginput density values representing the grey levels of various locationsof the original for a given color separation, defining a desired screendot arrangement for the image, writing screen dots in a line by linefashion, wherein each screen dot is made up of a plurality of lineswhose length and location determines the dot configuration and whoselength and location is determined by an analog operation employing theinput density values of the original and the desired screen dotarrangement.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved technique andapparatus for generating a screened reproduction of an image.

There is thus provided in accordance with a preferred embodiment of thepresent invention apparatus for generating a screened reproduction of animage comprising:

apparatus receiving input density values of an original and coordinateinformation and storing screening information for providing an operatinginput to a plotter enabling the plotter to provide a screened image ofthe original, the apparatus being characterized in that it includesapparatus for modifying at least one of the input density values, thecoordinate information and the screening information, thereby to providean operating input generally free of repeated artifacts arising from thelack of coordination between the coordinate systems of the plotter andthose of the screening information.

There is also provided in accordance with a preferred embodiment of thepresent invention apparatus for generating a screened reproduction of animage comprising:

a plotter

apparatus receiving input density values of an original and coordinateinformation and storing screening information for providing an operatinginput to the plotter enabling the plotter to provide a screened image ofthe original, the apparatus being characterized in that it includesapparatus for modifying at least one of the input density values, thecoordinate information the operational characteristics of the plotterand the screening information, thereby to provide an operating inputgenerally free of repeated artifacts arising from the lack ofcoordination between the coordinate systems of the plotter and those ofthe screening information.

Preferably the apparatus for modifying is operative to maintain auniform number of plotter lines in each dot for a given input density.

Preferably the apparatus for modifying is also operative to maintain thecenter of each dot at a uniform location within the dot for a giveninput density.

Additionally, the apparatus for modifying preferably is operative tomaintain the percentage of exposed area per dot uniform for each giveninput density.

Preferably the apparatus for modifying is operative to maintain auniform number of plotter lines in each group of dots for a given inputdensity.

Preferably the apparatus for modifying is also operative to maintain thecenter of each group of dots at a uniform location within the group ofdots for a given input density.

Additionally, the apparatus for modifying preferably is operative tomaintain the percentage of exposed area per group of group of dotsuniform for each given input density.

Further in accordance with an embodiment of the present invention thereis provided a method for generating a screened reproduction of an imagecomprising the steps of:

receiving input density values of an original and coordinate informationand storing screening information for providing an operating input to aplotter enabling the plotter to provide a screened image of theoriginal, the method being characterized in that it includes the step ofmodifying at least one of the input density values, the coordinateinformation and the screening information, thereby to provide anoperating input generally free of repeated artifacts arising from thelack of coordination between the coordinate systems of the plotter andthose of the screening information.

There is also provided in accordance with a preferred embodiment of thepresent invention a method for generating a screened reproduction of animage comprising receiving input density values of an original andcoordinate information and storing screening information for providingan operating input to a plotter enabling the plotter to provide ascreened image of the original, the method being characterized in thatit includes the step of modifying at least one of the input densityvalues, the coordinate information the operational characteristics ofthe plotter and the screening information, thereby to provide anoperating input generally free of repeated artifacts arising from thelack of coordination between the coordinate systems of the plotter andthose of the screening information.

Preferably the step of modifying is operative to maintain a uniformnumber of plotter lines in each dot for a given input density.

Preferably the step of modifying is also operative to maintain thecenter of each dot at a uniform location within the dot for a giveninput density.

Additionally, the step of modifying preferably is operative to maintainthe percentage of exposed area per dot uniform for each given inputdensity.

Preferably the step of modifying is operative to maintain a uniformnumber of plotter lines in each group of dots for a given input density.

Preferably the step of modifying is also operative to maintain thecenter of each group of dots at a uniform location within the group ofdots for a given input density.

Additionally, the step of modifying preferably is operative to maintainthe percentage of exposed area per group of group of dots uniform foreach given input density.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood and appreciated fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 is a simplified block diagram illustration of a process colorprinting system constructed and operative in accordance with a preferredembodiment of the present invention;

FIG. 2 is an illustration of one color separation of a typical colororiginal;

FIG. 3 is an enlarged illustration of a small portion of the halftonecolor separation of FIG. 2;

FIG. 4 is an illustration of pixel-by-pixel input density values for thesmall portion of the color separation of FIG. 2;

FIG. 5 is a generalized flow chart illustrating operation of theinvention;

FIGS. 6, 7 and 8 are more detailed flow charts, each showing in enhanceddetail, part of the operation of the invention illustrated in FIG. 5;

FIGS. 9A and 9B each show modification of a dot shape in accordance withthe teachings of the present invention;

FIGS. 10A and 10B each show modification of a group of dots inaccordance with the teachings of the present invention; and

FIG. 11 shows the modification of part of a screened image in accordancewith the teachings of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Reference is now made to FIG. 1, which illustrates a system forgenerating a screened reproduction of an image constructed and operativein accordance with a preferred embodiment of the present invention. Thesystem preferably comprises a color separation scanner 10, such as aScitex Smart Scanner, manufactured and sold by Scitex Corporation Ltd.of Herzlia, Israel, which is adapted to provide a digital colorseparation output of a color original.

The digital output of scanner 10 is normally stored on an image datadisk 12 or any other suitable storage medium, which is accessible by aCPU 14, such as an Intel 80486. Interfacing with the CPU 14 is aninteractive workstation 16, such as a Scitex Prisma, manufactured andsold by Scitex Corporation Ltd. of Herzlia, Israel.

CPU 14 interfaces with screen dot generation circuitry 18, which in turnprovides a control output to laser beam control circuitry 24 in a laserplotter 26, such as a Raystar, manufactured and sold by ScitexCorporation Ltd. of Herzlia, Israel. Dot generation circuitry 18 shouldbe constructed to permit variation of the dot shape as a function of theinput density and of the plotter coordinates. Dot generation circuitrywhich is suitable is described in Israel Patent Application 105343 filedApr. 4, 1993 and U.S. patent application Ser. No. 07/947,282, filed Sep.16, 1992.

Laser plotter 26 produces halftone film color separations 28 which areemployed in a conventional process color printing press 30, to produceprocess color prints.

Reference is now made to FIGS. 2 and 3. FIG. 2 illustrates a typicalhalftone color separation which is stored on disc 12. FIG. 3 illustratesin enlarged detail, a small area 32 indicated on FIG. 2. It is notedthat the gray level over area 32 varies thereacross.

The halftone color separation in general and the small area 32 inparticular are hereinafter termed the input image and are divided into afirst multiplicity of pixels 34 which are arranged along coordinate axesI_(x) and I_(y). Pixels 34 typically have a resolution of 100-400 pixelsper inch along each of the coordinate axes of the input image.

Each average gray level for a pixel 34 is represented digitally by aninput density level. There are typically provided 256 different inputdensity levels, 0 being the lightest and 255 being the blackest. FIG. 4illustrates the input density values for the pixels 34 of FIG. 3.

In accordance with a preferred embodiment of the present invention, theCPU 14 is operative to modify the dot shape produced by the dotgenerator 18 so as to eliminate repeated artifacts arising from the lackof coordination between the coordinate systems of the plotter 26 andthose of the screening information.

Reference is now made to FIG. 5 which illustrates the present inventionin general terms. Generally speaking, the invention comprises thefollowing principal functions:

A. calculating the average shape of one dot or a group of dots;

B. modifying the shape of the dot or group of dots to have greateruniformity in general and more particularly to have a uniform center ofarea coverage;

C. modifying the overall size of the dot or group of dots in order tomore closely match the theoretical dot size determined by the inputdensity of the original.

The foregoing steps are preferably carried out by software in the CPU,it being appreciated that the invention could equally have a hardwareimplementation.

Reference is now made to FIG. 6, which is a more detailed exposition ofstep A above.

The initial stage of step A is receiving the following parameters: InputDensity, Plotter Resolution, Screen Function, Screen Mesh and ScreenAngle.

The screen function is operated coordinate-by-coordinate to create anarray of dots of size M.

An averaging calculation is carried out on the array of size M to obtainan average dot shape, which is typically of size N, where N<<M. Thisarray of size N is employed hereinafter as the "average dot or group ofdots".

Reference is now made to FIG. 7, which illustrates step B. Each dot inthe array of size M is compared with the dot configuration in the arrayof size N, which represents the average dot or group of dots. Based onthe comparison, each of the dots in the array of size M is modified toapproach the configuration of the average dot or group of dots in thearray of size N.

Next, the thus modified dots or groups of dots of the array of size Mare inspected to determine their respective centers. Based on theresults of this inspection, the configurations of the modified dots orgroups of dots are further modified to approach a situation wherein thecenters of each dot and group of dots are uniformly located.

Reference is now made to FIG. 8, which illustrates step C. The twicemodified dots provided by step B are inspected to determine their area.On the basis of the results of this inspection, the areas of the twicemodified dots or groups of dots are further modified to provide dotswhich exhibit a generally linear change in dot size as a function ofinput density.

The result of steps A, B and C is an array of size M of thrice modifieddots, which are generally free of repeated artifacts arising from thelack of coordination between the coordinate systems of a plotter andthose of screening information used to generate a screen.

Reference is now made to FIGS. 9A and 9B which show examples of theoperation of the present invention. The dots at the left side of FIGS.9A and 9B have an unmodified configuration, while the dots on the rightside of FIGS. 9A and 9B have a thrice modified configuration, namely,their configuration, center location and size have been modified towardsuniformity.

Reference is now made to FIGS. 10A and 10B which show examples of theoperation of the present invention. The groups of groups of dots at theleft side of FIG. 10A and and top drawing of FIG. 10B have an unmodifiedconfiguration, while the groups of dots on the right side of FIG. 10Aand lower drawing of FIG. 10B have a thrice modified configuration,namely, their configuration, center location and size have been modifiedtowards uniformity. In both figures, the center of the theoretical groupof dots is indicated by reference numeral 40 and the center of theactual group of dots is indicated by reference numeral 42.

FIG. 11 illustrates the effects shown in FIGS. 9A and 9B over part of ascreen. Although the improvements due to the present invention cannotreadily be seen by an untrained person with the naked eye, it isappreciated that the dot array is significantly improved from thestandpoint of reduction in repeated artifacts.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather, the scope of the present invention isdefined only by the claims which follow:

I claim:
 1. Apparatus for generating a screened reproduction of an imagecomprising:apparatus receiving input density values of an original andcoordinate information and storing screening Information for providingan operating input to a plotter enabling the plotter to provide an arrayof dots forming a screened image of the original, each dot being formedby a number of plotter lines, said apparatus being characterized In thatit Includes apparatus for modifying at least one of the input densityvalues, the coordinate information and the screening information,thereby to provide an operating input generally free of repeatedartifacts arising from the lack of coordination between the coordinatesystems of the plotter and those of the screening information, whereinsaid apparatus for modifying is operative to maintain a uniform numberof said plotter lines in each said dot for a when input density. 2.Apparatus according to claim 1 and wherein said apparatus for modifyingis operative to maintain the center of each dot at a uniform locationwithin the dot for a given input density.
 3. Apparatus according toclaim 1 and wherein said apparatus for modifying is operative tomaintain the percentage of exposed area per dot uniform for each giveninput density.
 4. Apparatus for generating a screened reproduction of animage comprising:a plotter apparatus receiving input density values ofan original and coordinate information and storing screening informationfor providing an operating input to said plotter enabling the plotter toprovide an array of dots forming a screened Image of the original, eachdot being formed by a number of plotter lines, said apparatus beingcharacterized In that it includes apparatus for modifying at least oneof the Input density values, the coordinate information, the operationcharacteristics of the plotter and the screening information, thereby toprovide an operating input generally free of repeated artifacts arisingfrom the lack of coordination between the coordinate systems of theplotter and those of the screening information, wherein said apparatusfor modifying is operative to maintain a uniform number of said plotterlines in each said dot for a given input density.
 5. Apparatus accordingto claim 4 and wherein said apparatus for modifying is operative tomaintain the center of each dot at a uniform location within the dot fora given input density.
 6. Apparatus according to claim 4 and whereinsaid apparatus for modifying is operative to maintain the percentage ofexposed area per dot uniform for each given input density.
 7. A methodfor generative a screened reproduction of an image comprising the stepsof:receiving Input density values of an original and coordinateinformation and storing screening information for providing an operatinginput to a plotter enabling the plotter to provide an array of dotsforming a screened image of the original, each dot being formed by anumber of plotter lines, the method being characterized in that itincludes the step of modifying at least one of the input density values,the coordinate information and the screening information, thereby toprovide an operating input generally free of repeated artifacts arisingform the lack of coordination between the coordinate systems of theplotter and those of the screening information, wherein said modifyingstep operative to maintain a uniform number of said plotter lines ineach said dot for a given input density.
 8. A method according to claim7 and wherein said modifying step is operative to maintain the center ofeach dot at a uniform location within the dot for a given input density.9. A method according to claim 7 and wherein said modifying step isoperative to maintain the percentage of exposed area per dot uniform foreach given input density.
 10. A method for generating a screenedreproduction of an image comprising receiving input density values of anoriginal and coordinate information and storing screening informationfor providing an operating input to a plotter enabling the plotter toprovide an array of dots forming a screened image of the original, eachdot being formed by a number of plotter lines the method beingcharacterized in that it includes the step of modifying at least one ofthe input density values, the coordinate information, the operationalcharacteristics of the plotter and the screening information, thereby toprovide an operating input generally free of repeated artifacts arisingform the lack of coordination between the coordinated systems of theplotter and those of the screening information,wherein said modifyingstep is operative to maintain a uniform number of said plotter lines ineach said dot for a given input density.
 11. A method according to claim10 and wherein said modifying step is operative to maintain the centerof each dot at a uniform location within the dot for a given inputdensity.
 12. A method according to claim 10 and wherein said modifyingstep is operative to maintain the percentage of exposed area per dotuniform for each given input density.